We developed two new three-dimensional models of aerosol transport and deposition representative of two generations in the alveolar zone of the human lung, one with the bifurcation area, and one without. The models were used to simulate particle trajectories of 1-5 mu m particles in generations 18-22 of the human lung in different orientations with respect to gravity. Orientation had a significant effect on the overall deposition predicted, which varied by a factor of approximately 3 between the orientations with highest and lowest deposition. Total deposition was a function of orientation and the ratio of terminal settling velocity to mean lumen flow velocity. Predicted deposition was significantly greater in the model without the bifurcation than in the model with the bifurcation. We conclude that modeling the bifurcation is important due to the complex relationship between aerodynamic drag and gravitational sedimentation within the bifurcation zone. (C) 2005 Elsevier Ltd. All rights reserved.